What i love from a more linear aspect of RPGs is the hype built from the Enemy/Antagonist ventures & theoretical progress/exposition in the world at play. Kefka was a sick bastard due to his impulsive destructive behavior that led to the end of the world. Imagine if there was a field of Kefkas spawning in a field out somewhere, where you had to fight him because of the lust of a sword you get as a end-goal?
I don't think you can really compare these two things across the board. Kefka is a plot intensive NPC, and MMORPGs have those too. In World of Warcraft there is only one Onyxia who, much like Kefka, is built up through lore, dies in a cutscene in a closed linear environment, etc...
There are (beloved) linear single player RPGs with stuff you have to mindlessly grind for, too; that's not limited to MMORPGs. Earthbound has a lot of items that have only a 1/128 chance of dropping. I do understand what you mean when you say MMORPGs have an abundance of it, though.
(edit: also, I love outlaw star!!)